Histology of Lymphoid Organs

These flashcards cover key vocabulary related to the histology of lymphoid organs and their roles in the immune system.

Lymphatic Organs

Organs involved in the production and maturation of lymphocytes, including primary (thymus, bone marrow) and secondary (lymph nodes, spleen).

Thymus

Primary lymphatic organ where T cells mature; includes cortex and medulla, and features Hassall's corpuscles.

Blood-Thymus Barrier

Barrier that protects developing T cells in the thymus from exposure to antigens.

Lymph Nodes

Small, bean-shaped organs that filter lymph and initiate immune responses; contain afferent and efferent lymphatic vessels.

Lymphatic Vessels

Transport lymph (fluid containing lymphocytes) back to the bloodstream and are more permeable than blood capillaries.

Diffuse Lymphatic Tissue

Non-discrete collections of lymphocytes in mucosa, important for immune response.

Lymphatic Nodules

Discrete collections of lymphocytes within reticular tissue that filter lymph and expose lymphocytes to antigens.

B Cells

Lymphocytes that mature in the bone marrow and are involved in humoral immunity by producing antibodies.

T Cells

Lymphocytes that mature in the thymus, involved in cell-mediated immunity.

NK Cells

Natural killer cells, part of the innate immune system that directly kill infected or transformed cells.

Spleen

Largest lymphatic organ that filters blood, rich in lymphocytes, and has both red and white pulp.

Germinal Centers

Areas within lymphatic nodules where B lymphocytes proliferate after encountering an antigen.

Primary Immune Response

The body's first encounter with an antigen, characterized by a lag period before specific lymphocytes are detected.

Secondary Immune Response

More rapid and intense response that occurs upon subsequent encounters with the same antigen, involving memory B cells.

Hassall's Corpuscles

Structures found in the medulla of the thymus, associated with the maturation of T cells.

Reticular Cells

Cells that create a supportive framework in lymphoid tissues and secrete reticular fibers.

What is the primary role of primary lymphatic organs?

Primary lymphatic organs, such as the thymus and bone marrow, are responsible for the production and maturation of lymphocytes (B and T cells).

What is the primary role of secondary lymphatic organs?

Secondary lymphatic organs, including lymph nodes and the spleen, are sites where mature lymphocytes encounter antigens and initiate immune responses.

What are shared features of lymphoid organs?

Common features of lymphoid organs include a reticular connective tissue framework, the presence of various lymphocytes, and a role in immune surveillance or response.

What is a unique feature of the thymus?

The thymus is unique due to its role in T cell maturation and selection, and the presence of Hassall's corpuscles in its medulla.

What is a unique feature of lymph nodes?

Lymph nodes are unique for filtering lymph via afferent and efferent lymphatic vessels, serving as sites for immune activation from tissue-borne antigens.

What is a unique feature of the spleen?

The spleen is unique as the largest lymphatic organ, responsible for filtering blood and featuring distinct red and white pulp regions.

What are some key functions of the thymus?

The thymus functions as the primary site for T cell maturation, involving positive and negative selection processes to ensure T cells are immunocompetent and self-tolerant.

How do lymphatic vessels differ from blood capillaries?

Lymphatic vessels are typically blind-ended, lack a continuous basement membrane, are generally more permeable than blood capillaries, and transport lymph only away from tissues. They also contain more valves due to lower pressure.

Describe the morphology of a lymph node.

Lymph nodes are encapsulated, bean-shaped structures with outer cortex (containing lymphatic nodules with germinal centers and B cells), a paracortex (rich in T cells), and an inner medulla (containing medullary cords and sinuses).

What are the primary functions of lymph nodes?

Lymph nodes filter lymph, removing antigens and cellular debris, and serve as crucial sites for initiating adaptive immune responses by exposing lymphocytes to processed antigens presented by antigen-presenting cells.

What are the similarities between diffuse lymphatic tissue and lymphatic nodules?

Both diffuse lymphatic tissue and lymphatic nodules are collections of lymphocytes within reticular tissue, involved in immune responses, and are found in various mucosa-associated lymphoid tissues (MALT).

What are the key differences between diffuse lymphatic tissue and lymphatic nodules?

Diffuse lymphatic tissue consists of loosely organized lymphocytes without distinct boundaries, while lymphatic nodules are discrete, often spherical, and can contain germinal centers, indicating active B cell proliferation.

What are the main functions of the spleen?

The spleen filters blood, removes old or damaged red blood cells and platelets (red pulp function), and initiates immune responses against blood-borne pathogens (white pulp function).

How can white pulp be differentiated from red pulp in histologic sections of the spleen?

In histologic sections, \textbf{white pulp} appears as basophilic (darker staining) areas rich in lymphocytes surrounding central arterioles, forming periarteriolar lymphoid sheaths (PALS) and lymphatic nodules. \textbf{red pulp} appears as eosinophilic (lighter staining) areas composed of splenic cords (cords of Billroth) and venous sinuses, responsible for blood filtration and erythrocyte destruction.